TRANSITION-STATES, AVOIDED CROSSING STATES AND VALENCE-BOND MIXING - FUNDAMENTAL REACTIVITY PARADIGMS

A chemical model has been constructed for the transition state (TS) th at is otherwise defined only by mathematical terms as a saddle point o n the potential-energy surface. The proposed model is the avoided cros sing state (ACS) of the chemical reaction. Unlike the TS that is a pri ori unknown, the ACS possesses a wavefunction that is prescribed by th e constraints of the avoided crossing and is explicit in terms of the participating VB configurations. These VB configurations provide simul taneously a generalized TS description along with lucid information ab out the chemical nature of the TS. Ab initio computations demonstrate that, for nine S(N)2 and nucleophilic addition reactions, the ACS is a n excellent approximation for the TS. This proximity between the two s tructures means in turn, that the bottleneck of the reaction may be as sociated with the chemically well defined ACS. VB mixing ideas are use d to articulate the ACS paradigm and derive electronic properties of t his state and its antibonding companion state. Applications to ground- state and excited-state reactivities of electrophile-nucleophile combi nations are discussed.